An exhaust gas after treatment system for a modern diesel engine exhaust system typically incorporates a selective-catalytic-reduction (SCR) device. The SCR device is utilized to reduce NOx gases. The SCR device uses a reductant capable of reacting with NOx gases to convert the NOx gases into inert byproducts, i.e., nitrogen and water.
For example, the reductant can be an aqueous solution of urea, which is injected into the engine's exhaust stream. Once the reductant is in the exhaust stream, the reductant is absorbed into the SCR device, where the catalytic action of the SCR device ultimately converts NOx gases into the inert byproducts. Conventionally, urea is injected into the exhaust stream at temperatures above 250° C. to reduce the accumulation of urea deposits within the exhaust gas system.